英文是：on a diet。
解释：
on a diet 英[?n dai?t] 美[ɑn e ?dat]
[词典] 节食;
[例句]Have you been on a diet? You've lost a lot of weight
你最近在节食吗？你瘦了好多。
He has been on a diet for as long as any of his friends can remember
在朋友的记忆中，他一直都在节食。
I'm forever on a diet, since I put on weight easily
我永远都在减肥，因为我很容易长胖。
He lived on a diet of water and tinned fish
他仅靠喝水、吃鱼罐头过活。
'He'll have to go on a diet,' Ballard quipped
“他得减肥啦，”巴拉德俏皮地说。